A multi-client 2-D seismic survey, consisting of 6,770 line-kilometer of 2-D data, was acquired by Norway’s PGS Geophysical offshore Cyprus in 2006. It was followed in 2006 and 2007 by the acquisition of two 3-D surveys, located offshore Lebanon (1,500 square kilometers), and offshore Lebanon and Cyprus (1,300 square kilometers). Further, in 2008 an additional 2-D survey was acquired offshore Cyprus to fill in the existing 2006 seismic 2-D grid. The 2008 2-D survey consist of 6,500 line-kilometer and was acquired with the PGS dual sensor GeoStreamer® technology (The 2008 2D data is still being processed so no GeoStreamer® examples are shown in this article). Since 2007, some of the deep-water areas (1,500–2,000 meters deep) offshore Cyprus have been offered for exploration and producing bid licenses by the government, while the Lebanese government is in the stage of preparing its offshore region for exploration and licensing.
In this paper, several representative seismic lines are shown to illustrate the quality of the seismic data over this large region. The data shows that a Pliocene to Recent succession overlies a thick Upper Miocene evaporite section, known as the Messinian Salt. The blanketing salt-dominated interval is up to one kilometer thick and provides a regional seal over the area; although some faults are known to penetrate the salt and may allow seepage to occur. The pre-Messinian Salt section is highly structured, with both horsts-and-grabens as well as large folds. In particular, several examples of seismic reflections are presented, which are interpreted as possible Direct Hydrocarbon Indicators (DHI). Some four-way structural closures are highly prospective as they are characterized by multiple DHIs; for example, a bright spot at the base of the Messinian Salt overlying a flat spot. Other examples consist of phase changes, dim spots, velocity pull-downs, low-frequency shadow zones and gas chimneys. Also shown are how some of these interpreted DHIs can be characterized in the 3-D surveys. The 3-D example illustrate a correlation between pre-Messinian four-way structural closure and the root-mean square amplitude of the base Messinian Salt reflection.
The offshore regions of Cyprus and Lebanon in the easternmost Mediterranean Sea (Figure 1, Bremen, 2006, inRoberts and Peace, 2007; Elias, 2007) are currently unexplored for petroleum resources – but this is set to change in the near future. Since 2007, Cyprus has conducted bid rounds for their offshore exploration and producing blocks (Figure 2), which are located close to the proven hydrocarbon-producing provinces of Egypt’s offshore Nile Delta Cone and near-offshore basins of the Levantine Basin in the southeastern Mediterranean Sea.
These deep-water basin areas, which contain a Mesozoic – Cenozoic sedimentary succession up to 10 km thick, are prospective with large structures, a regional seal and an active petroleum system (see review inRoberts and Peace, 2007). According to Oil and Gas International (Oil and Gas International, Cairo, January 26, 2007) previous studies have been encouraging, estimating that oil and natural gas reserves in the seas surrounding Cyprus amount to 6 to 8 billion barrels of oil equivalent. It is therefore not surprising that the area is already receiving attention from the oil industry. This paper shows examples of recently acquired high-quality seismic data that clearly image several large structures, with associated potential Direct Hydrocarbon Indicators (DHI), which supports the prospective evaluation of the region.
DATA ACQUISITION AND PROCESSING
Norway’s PGS Geophysical acquired a multi-client 2-D survey (Figure 2), consisting of 6,770 line-kilometer of 2-D seismic, in 2006 in offshore Cyprus. The survey was acquired with the vessel M/V Falcon Explorer using a 4720 cubic inch air gun at 8 meter and an 8,100 meter long streamer with 9.2 seconds recording length. The acquisition geometry and processing flow was optimized to image beneath the salt as well as image the Eratosthenes seamount (Figure 1), which has a different geology than the Cyprus Thrust Zone and the Levantine Basin (Robert and Peace, 2007).
The main seismic processing steps were: (1) minimum phase conversion; (2) attenuation of swell noise; (3) spatial anti-alias filter; (4) first-pass demultiple using Surface Related Multiple Attenuation (SRME); (5) Radon demultiple deconvolution; (6) 2-D Kirchhoff pre-stack time migration (PSTM); (7) residual moveout correction; and (8) post-stack processing.
In addition, PGS acquired two multi-client 3-D surveys in 2006 and 2007 in offshore Cyprus and Lebanon waters (1,500 square km in the Levantine Basin, and 1,300 square km in the Cyprus Basin, Figure 2). The vessel M/V Atlantic Explorer used six streamers and two 3,090 cubic inch air guns positioned at a depth of 6 m. The streamer length was 6,000 m and the streamer separation 100 m. The processing flow was optimized to image below the salt with focus on true-amplitude preservation.
The main processing steps included: (1) linear noise attenuation; (2) velocity analysis (initial velocities); (3) first-pass demultiple (SRME); (4) second-pass demultiple (Radon deconvolution); (5) zero-phasing and Q-compensation; (6) velocity analysis (migration velocities); (7) 3-D Kirchhoff pre-stack time migration (PSTM); (8) third-pass demultiple (Radon deconvolution); and (9) stacking and post-stack processing.
EXAMPLES OF REGIONAL LINES
Figure 3 and 4 show two seismic lines that illustrate the general quality of the seismic 3-D data. North-south Line A (Figure 3) was acquired in water depths of approximately 1,700 m and is perpendicular to the Lebanese coastline, about 50–60 km offshore Beirut. Beneath the ocean bottom, a continuous Pliocene – Recent sedimentary package overlies the Mesinnian Salt sequence. The package thickens towards Lebanon and blankets most of the Levantine Basin. This sequence is believed to consist of turbidities that were sourced from mountains in Lebanon (Beydoun, 1977). Below the package, the Messinian evaporite sequence (Messinain Salt) is about one kilometer thick and dominated by rock salt. It can be mapped together with some more-or-less continuous intra-Messinian packages, which are interpreted as reflections from gypsum, anhydrite and clastic beds.
The Messinian Salt provides a regional seal in the region, below which large four-way-closure structures are evident in the Tertiary and Cretaceous section. One such structure is imaged to the left in Line A (Figure 3). Below the Messinian Salt, normal faults segment the section into horsts and grabens that may contain hydrocarbon traps. Many of the faults terminate below the Messinian Salt implying an extensional event occurred in the Early Tertiary. Other faults, however, also appear to continue through the salt and into the Pliocene – Recent succession, seen as breaks both in the intra-Messinian reflections and Pliocene – Recent succession (e.g. Line A southern part and Line B western part, Figure 4). Indeed an anonymous reviewer noted that “often large-scale, deep-seated faults penetrate right through the Messinian Salt and to the seabed, some with associated DHI and seepages evident. There are likely other shale seals as well. Other faults just penetrate from seabed down through the salt in which they usually sole out, at or around base salt.”
Further down in the seismic section, a reflection is here interpreted as the Senonian unconformity (i.e. top Jurassic). According to an anonymous reviewer this pick is highly debated by most interpreters because no direct well control is available. He reported that some interpreters have picked it as high as their intra-Tertiary and as low as my base Cretaceous pick.
West–east Line B was acquired further to the north covering partly Cyprus and partly Lebanon (Figure 4). It was extracted from the middle of the northern 3-D survey, which was recorded in water depths of approximately 1,500 m. In contrast to Line A (Figure 3), Line B images extensive folds originating from the deep pre-Cretaceous section. These may have started to form in the Late Cretaceous (Turonian) with the emplacement of the Todroos Ophiolite and related thrust systems of Cyprus and persisted until the Recent. The extensively folded zone, named the Latakia Ridge, pierces the sea bottom and stands about 400 m above it. It continues to the west where it connects to the Cyprus Thrust Zone (also known as the Larnaka-Latakia Ridge System) along the southern part of Cyprus. This zone accomodates subduction related to the collision between the north-drifting Africa, Arabian and Levant plates and the Turkish (Anatolian) Plate. The Latakia Ridge is a morphological high feature that formed by a combination of a subduction and left-lateral, strike-slip movements.
DIRECT HYDROCARBON INDICATORS
Direct Hydrocarbon Indicators (DHI) have been successfully used to identify the occurrence of hydrocarbons from reflection seismic data for over 30 years. The methods originated primarily during the exploration of Upper Tertiary clastic reservoirs. It has since evolved into a tool that can be used in virtually every geological province (Rice et al., 1981). The most commonly identified DHIs include flat spots, bright spots, dim spots, phase changes, velocity push-downs, low-frequency zones, gas chimneys and shallow high-amplitude zones. DHIs provide one of the simplest methods for identifying prospects, especially when they occur together with seismically defined structural and stratigraphic traps.
Even though DHIs are widely used, great caution is required when analyzing a prospect. An event that appears as a DHI might be due to an abrupt lateral lithological variation, diagenetic effects, large pressure change or unreliable seismic amplitude information. In cases where a dry hole cannot be ascribed to any of these causes, it is often due to low gas saturation (O’Brien, 2004). An anonymous reviewer noted correctly that: “even a small change (a few percentages in gas presence) can produce a DHI effect, which is just a gas show but not a commercial discovery. Unfortunately the quality of seismic does not always improve the quality of DHI reliability.” It is therefore important to evaluate the geological risk by also considering the geological setting of the DHI to assess whether an amplitude anomaly might represent hydrocarbons (Roden et al., 2005).
DHI prospects may be interpreted and assessed differently from company to company. For additional proof of hydrocarbon presence, a DHI prospect should be assessed by combining information from other analyses, e.g. amplitude-versus offset (AVO) studies.
Seismic Flat Spot
A seismic flat spot may be a reflection from a well-defined fluid contact, commonly the gas/oil or gas/water contacts. The acoustic impedance contrast between the two phases may be sufficiently large to produce a strong reflection. In a section with dipping reflections, it stands-out because of its flat attitude. This is usually taken to be the most definitive and informative of all the Direct Hydrocarbon Indicators (Sheriff, 1995). A good example is the ultra-deep water Shell gas discovery in Well Kg 45-1b in Egypt’s offshore Nile Delta NEMED block, where a flat spot was identified in the Pliocene, as clearly imaged in the 2-D seismic data (Figure 5).
In offshore Cyprus, flat spots occur in the Cretaceous and Tertiary successions in several areas: (1) to the northwest of the Eratosthenes Seamount; (2) south of the Cyprus Thrust Zone; and (3) below the Messinian Salt in the Levantine Basin. In offshore Lebanon, flat spots have been identified from 2-D and 3-D seismic surveys in Cretaceous and Tertiary sequences in the Levantine Basin.
In Figure 6 an example of a Pliocene flat spot and two bright spots are shown from offshore Cyprus in the western part in the Herodotus Basin (Figure 1). These DHIs are believed to occur in the same stratigraphic interval as in Shell’s discovery but were not directly tied to Shell’s survey or wells in the Nile Delta cone (Figure 5). Figure 7 shows another example of a flat spot imaged below the Messinian Salt and above a large structural closure.
Seismic Bright Spot
A seismic bright spot is a strong-amplitude reflection caused by large changes in acoustic impedance and tuning effects. In general, bright spots are mostly caused by lateral changes in lithology rather than DHIs. Nevertheless bright spot DHIs can also be due to a gas-saturated sandstone reservoir underlying a shale interval (Figure 5). When seismic bright spots are on top of a structural high they are often associated with gas accumulation as implied in Figure 6. Gas-induced bright spots usually have negative polarity for the reflection from the top of the reservoir.
An anonymous reviewer added: “Bright spots may have several causes and no general rule is likely to apply. In siliciclastic rocks with intermediate to high porosity (15−30%), bright spots may be caused by gas in both good and poor reservoirs, with high or low gas saturations. In this setting, low gas saturations (5−10%) may give a similar response compared to a case with high saturation, in addition often also giving erroneous positive results in the AVO analysis. In carbonates and low porosity siliciclasics, bright spots are more likely to reflect lithological changes, primarily porosity.”
Seismic Dim Spot
By contrast to a bright spot, a seismic dim spot shows weak rather than strong amplitudes. The weak amplitude can correlate to the presence of hydrocarbons that reduce the contrast in acoustic impedance (AI) between the reservoir and the overlying rock. Such dim spots are often associated with the occurrence of oil or gas.
To determine whether any spatial relationships occur between seismic amplitude and exploration prospects the reflection amplitude values from the base Messinian Salt reflector were extracted. This reflector was chosen because its amplitude variation may be related to the presence of hydrocarbons trapped immediately below the salt. It is also possible that any such variations or apparent flat spots may only represent salt-related geological changes (e.g. flat base salt) rather than gas indicators. Nevertheless, the root mean square (RMS) amplitude values in Figure 8 show a good conformance of amplitude and structure within the identified closures.
Phase Changes in Seismic Data
In some cases the thickness of a reservoir may be small compared to the resolution of seismic data. In such cases reflections from the reservoir caprock, fluid contact, and the base of the reservoir generally interfere with one another. The resulting composite reflection may show various phase and amplitude changes due to the interference, and in different ways. This may be regarded as a potential hydrocarbon indicator (Sheriff, 1995). Figure 9 shows areas with pronounced dimmed amplitudes approximately parallel to the trend of the Syrian Arc in the Levantine Basin. The dimmed amplitude could be related to a hydrocarbon-induced phase changes, but also to other geological changes.
Seismic Gas Chimneys and Velocity Pushdown
A gas chimney describes the effects of escaped gas that is dispersed upwards in the sediments as imaged in the seismic data. The presence of the gas causes the seismic reflections to abruptly become dim or altogether disappear in the zone. Dimmed zones are observed on the seismic data in several locations in the survey, mostly within the Pliocene strata. In these particular instances, the Pliocene turbidities, outside the noisy zones are imaged by well-defined and continuous reflections. However they break-up or can not be correlated into the noisy gas chimney zone.
The reduction of velocity through a hydrocarbon accumulation will also affect reflections from deeper intervals by increasing the two-way times. This is because the accumulation has a lower seismic velocity that causes the reflections to sag (Sheriff, 1995). This low-velocity trend in the noise zones is characteristic of all the gas chimneys or noise zones seen in the seismic data from offshore Lebanon and Cyprus (Figure 10).
The seismic line in Figure 10 images a geological feature with a complex set of geological and seismic attributes. Immediately below the sea bottom reflection a c. 2.0-km-wide, high-amplitude reflection package is interpreted as Pliocene – Quaternary infill sediments (centered at about 2.0 second). This package has an isochron of about 0.2 seconds corresponding to some 200–300 meters of infill. Below this feature the section is chaotic and may represent a combination of faulting, dissolution of salt and a gas chimney. The zone is nearly absent of reflections although the deeper pre-salt Cretaceous – Tertiary reflections are pushed down towards the feature (Figure 10b).
It is unclear as to whether the push-down is due to hydrocarbons being present in the poor data zone (gas chimney) or a seismic artifact. In the latter case the push-down would be due to the replacement of high-velocity salt with low-velocity infill sediments. In this case pre-stack depth migration (PSDM) using accurate shallow velocities is required to correct for ray-path distortions and to better understand this feature.
Low-Frequency Seismic Shadow Zones
A lowering of the seismic instantaneous frequency is often observed immediately beneath hydrocarbon accumulations. Such low-frequency seismic shadows seem to be confined to a couple of cycles below accumulations. One anonymous reviewer invoked attenuation of the higher frequencies through the hydrocarbon zone as an explanation for this phenomenon; in particular for gas this effect can be quite large. The second anonymous reviewer further suggested that the removal of higher frequencies may be due in part to improper stacking with erroneous velocity assumptions or ray-path distortions (see Sheriff, 1995).
Several low-frequency zones are seen in the pre-Messinian Salt sediments of Cretaceous to Tertiary age. For example, a low-frequency and low amplitude shadow zone is imaged beneath a large four-way-dip structure located west of the Latakia Ridge (Figure 11). Another example is illustrated in Figure 12 where a bright spot at 3.0 seconds occurs below a low-frequency shadow zone in a large structure.
Example of Multiple Hydrocarbon Indicators
Figure 13 shows an example extracted from 3-D seismic data with several possible hydrocarbon indicators at different stratigraphic levels. Within the shallow Pliocene – Recent section a high-amplitude zone occurs above a four-way-dip structure at base and top of the Messinian Salt. The bright reflection zone occurs above a flat spot at about 2.5 seconds. Taken together, the flat spot may be a gas/water contact and the overlying bright zone a gas accumulation. The shallow gas accumulation may be associated with leakage from the deeper structure beneath the Messinian Salt.
The deep-water areas in offshore Cyprus and Lebanon are under-explored, frontier basins located close to several proven hydrocarbon systems. Both existing 2-D and newly acquired high-quality 3-D seismic data clearly image numerous large four-way structural closures with associated DHIs. These preliminary observations indicate that the area probably has a significant undiscovered resource base, which merits further exploration by oil and gas companies.
The author thanks the Cypriot Ministry of Industry, Tourism and Commerce and the Lebanese Prime Minister Office and Ministry of Energy and Water for permission to undertake seismic acquisitions in their territorial water. I also would like to thank my colleagues in PGS Caroline Jane Lowrey and Ørnulf Lauritzen for discussions and help. The comments by the two referees and Moujahed Al-Husseini are appreciated. The author thanks Nestor Buhay II for designing the final graphics.
ABOUT THE AUTHOR
Per Helge Semb obtained an MSc in Geophysics from the University of Oslo, Norway, in 2002. After graduating he joined PGS. From 2002 to 2005 he worked with four-component seismic processing including the supervision of the processing of the first four-components “Life of Field Project Valhall” project. Since 2005 he has been with PGS in the Middle East group working with development of new areas and in particular the Eastern Mediterranean and the Levantine basin.